GB2046552A - Improvements in radar detection devices - Google Patents

Abstract

In order to enable the beam transmitted by a radar detection device to emerge with a wide angle of divergence, the device is provided with a reflector (16) opposite a dielectric aerial (13) in order to deflect and spread out the beam emitted by a microwave transmitter/receiver module (11). The reflector (16) may be in the form of a metal half-cone having its apex (17) pointing towards the module (11) or alternatively in the form of a complete metal cone. Where the device is a motion detecting device of the kind used for detecting the presence of intruders in premises, this enables a wide area to be covered by the device. <IMAGE>

Description

SPECIFICATION Improvements in radar detection devices The invention relates to radar detection devices and particularly to motion detecting devices of the kind which transmit a microwave signal beam and receive signals reflected back from objects in the path of the beam, sensing from the reflected signals the presence of an object which is changing its distance from the device, i.e. which is moving towards or away from the device.

Such devices are commonly used to detect the presence of intruders in premises, or in specified areas of premises, and are arranged to activate an audible and/or visual warning device, or to put into effect some other desired operation, when the presence of a person moving towards or away from the detector is sensed within the field covered by the microwave beam. In such uses the beam is normally transmitted horizontally.

A disadvantage of known detection device of this kind is that the field covered by the beam transmitted by the device is limited in area. Typically, the main beam may emerge from the device with an angle of divergence of no more than 50 -55 , although some stray signals may also emerge from the device at other angles due, for example,. to random reflection from components within the device and associated with it. Consequently, in premises of large area it is frequently necessary to employ a number of detection devices in an endeavour to cover all possible attempts at access to the premises However, besides being costly, this still means that there are likely to be "blind" areas into which no signals are transmitted-and within which an intruder may move undetected.Also, it is possible for an important area of the premises to be turned into a "blind" area by displacement of one of the detection devices, either accidentally, or deliberately by someone who has occasional authorised access to the premises and wishes to return, unauthorised,-at some other time. The present invention sets out to provide an improved detection device which overcomes these disadvantages.

According to the invention there is provided a radar detection device comprising means for transmitting a microwave signal beam, means for receiving signals reflected back from objects in the path of the beam, means for sensing, from the reflected signals, the presence of an object which is changing its distance from the device, and reflecting means disposed in the path of the signal beam and shaped to deflect the signal beam in a manner to increase the angle of divergence thereof, thereby increasing the area of the field covered by the beam.

Preferably the reflecting means has a continuous curved surface the generators of which are inclined at an angle to the axis of propagation of the beam.

Thus, as the beam is reflected by the surface the curvature thereof causes the beam to be spread out so as to cover a greater area.

The said curved surface may comprise part of the surface of a cone. In this case the cone of which the curved surface forms part preferably has an axis which extends in the same direction as the axis of the beam.

In one embodiment of the invention the angular extent of said curved surface with respect to the axis of the cone is substantially 180 . With such an arrangement the angle of divergence of the beam, after reflection by the part-conical surface, is also substantially 180 and is thus significantly greater than the angle of divergence before reflection.

In a further embodiment of the invention, the angular extent of said surface with respect to the axis of the cone is substantially 360 . In this case the angle of divergence of the microwave beam after reflection is also substantially 360 , that is to say the beam extends radially outwardly in all directions. In this case the axis of the cone of which the reflecting surface forms a part preferably coincides with the axis of the beam so that the beam is reflected substantially uniformly outwardly of said axis.

In any arrangement where the reflecting surface forms part of the surface of a cone it may extend up to the apex of the cone. Thus in the case where the angular extent of the surface is 180 , the surface is that of a cone bisected along its central axis, i.e. a half-cone, whereas in the case where the angular extent of the surface is 36cur, the surface is simply the surface of a whole cone.

After deflection of the beam by the reflecting surface, the angle which the spread beam makes with respect to the beam axis before reflection will depend on the cone angle. Any appropriate cone angle may be used although it is found that the semi-vertical angle of the cone is preferably 45 or 6cur. In the former case it will be appreciated that the beam, after reflection, will spread out in a direction generally at right angles to the axis of the beam before reflection.

The following is a more detailed description of various embodiments of the invention reference being made to the accompanying drawings in which: Figure lisa perspective view of a radar motion detection device in accordance with the invention, the outer cover of the device being removed, Figure 2 is a side elevation of the device of Figure 1,showing also, in dotted lines, an alternative embodiment of the invention, Figure 3 is a diagram showing horizontal coverage patterns of detection devices in accordance with the invention compared with the coverage pattern of a conventional device, and Figure 4 is a block diagram of the detection circuitry of the device of Figures 1 and 2.

Referring to Figure 1, the detection device comprises a support frame 10 on which is mounted a microwave transmitter/receiver module 11. The module 11 may be of the known kind incorporating separate oscillator and mixer cavities in a single wave guide structure so that the resulting antenna radiation pattern is identical for both transmission and reception. The semi-conductor devices (not shown) incorporated in the module are retained in their holders by finger grip screw caps 12.

Mounted on the module 11 is a transmitting aerial which may take the form of a solid wedge of dielectric material 13, as shown in Figure 1, or a rectangu lar cross-section diverging horn 14, as shown in dotted lines in Figure 2. The module 11 produces a microwave beam, usually at a frequency between 9.3 and 10.7 GHz, having a range of some thirty to fifty metres.

The module 11 transmits the microwave signal beam and receives signals reflected back from objects in the path of the beam. The electronic circuitry associated with the module senses, from the transmitted and reflected signals, the presence of an object which is moving towards or away from the detector, i.e. which is changing its distance from the device. The circuitry is then arranged to activate an audible and/otvisual warning device, or put into effect some other desired operation, when the presence of an object moving towards or away from the detector is sensed within the field covered by the beam.

Referring to Figure 4, the signals received from the detector diodes of the transmitter/receiver module 11, when a moving object is detected, are each amplified by 40 db in each of three 40 db amplifier stages 40,41 and 42 and the amplified signals, which are of generally sinusoidal form, are supplied to wave shapers 43 and 44. The resultant generally square wave signals appearing at the outputs of the wave shapers 44 are supplied to a phase detector 45 in which the phases of the two signals are continuously compared. When the relative phases of the two signals indicated that the object is moving towards the device, an output signal is emitted from a first output of the detector 45 and a forward motion indicator lamp 46 is illuminated.When the relative phases of the two signals indicate that the object is moving away from the device, an output signal is emitted from a second output of the detector 45 and a reverse motion indicator lamp 47 is illuminated.

The level of the signals received from the detector diodes when an object is moving is sensed by a level detector 48 comprising a variable gain amplifier 49 whose gain may be adjusted between 0 and 40 dB, and two further wave shapers 50 and 51. This detector 48 emits a doppler burst output signal when its input exceeds a set threshold value. This output signal is applied to one or other of two continuous motion detectors 54 and 55 when one or other of two AND gates 52 and 53 is enabled by the presence of an output signal from the phase detector 45 indicating forward or reverse motion.Only when a signal indicative of motion in one direction is supplied con tinuouslyto one of the detectors 54 and 55 for a length of time greater than a predetermined value is an output signal emitted by an OR gate 56 with the result that a relay circuit 57 is triggered, thereby activating the warning device.

Normally the detection device is arranged with the longitudinal axis of the module 11 horizontal and a typical horizontal coverage pattern provided by the beam is shown at 15 in Figure 3.

It will be apparent from a consideration of Figure 3 that, where a large area is to be protected against intruders, a number of detectors must be provided to provide the necessary coverage. Frequently, however, overall cover is not practical and detectors are merely placed at strategic points where access to the area to be protected is most likely. Thus the detectors will not sense the presence of an intruder who succeeds in finding a way into the area from an unexpected quarter. Also, as previously mentioned, a person having occasional authorised access to an area may take the opportunity of adjusting the posi- ffi tion of a detector so that it no longer covers a path of access to the area thus enabling the person to return later, undetected. Also, of course, it may happen that the detector is accidentally moved out of position.

In the detection devices, according to the invention, shown in Figures 1 and 2, however, these disadvantages are reduced or eliminated. The devices according to the invention are provided with a reflector opposite the wedge or horn aerial to deflect and spread out the microwave beam transmitted by the module.

As shown in Figures 1 and 2 the reflector 16 may be in the form of a metal half-cone having its apex 17 pointing towards the module 11 and its axis spaced from and parallel to the longitudinal axis of the module. In operation the detector is normally mounted with the longitudinal axis of the module vertical with the reflector 16 either uppermost or lowermost. The microwave beam from the aerial 13 impinges on the conical surface of the reflector 16 and is turned through an angle (which depends on the cone angle of the reflector) and is spread out over an area which depends on the angular extent of the conical surface of the reflector 16 around its axis.

In the arrangement shown, the half-cone reflector has a semi-vertical angle of 45 . Consequently the beam is deflected through substantially 90 so that when the axis of the module is vertical the beam spreads out generally horizontally at an angle of divergence of about 180 as indicated at 18 in Figure 3.

In the alternative and preferred arrangement shown in dotted lines in Figure 2, the reflector 19 is in the form of a complete metal cone with its apex 20 pointing towards the aerial 13 or 14 and with its axis aligned with the longitudinal central axis of the module 11. In this case the microwave beam is spread out through 360 , i.e. it covers a complete circular area surrounding the detector, as indicated at 21 in Figure 3.

Although 45" semi-vertical angles are shown for the reflectors to produce a substantially horizontal spread of the beam when the detector is vertically arranged, other cone angles may be employed so that the beam coverage surrounding the detector has itself a generally conical configuration. In such cases the overall range in the horizontal direction will be reduced and there is indicated at 22 in Figure 3 a typical beam coverage where the reflector 19 has a semi-vertical angle of 60 instead of 45".

Using a 360 detection device, a single detector may be placed centrally in the area to be protected and will detect movement of a person anywhere within that area. Since the beam totally surrounds the detector it is not possible, as was the case with the known detectors, for a person to approach the detector on the "blind" side thereof to disable or displace it.

Since the detectors are normally arranged to detect only movement towards or away from the module it is possible, with known forms of detector, to move undetected across the beam by moving in a path at a constant distance from the detector. However, with the beam spread given by the detector according to the invention, movement in any direction into the beam will have a component towards or away from the detector so that such movement is bound to be detected.

Conveniently the detector may be centrally disposed close to the ceiling of a room to be protected and in this case it is preferable to so select the angle of the cone, as mentioned above, that a downwardly conical beam spread is achieved, the required angle of conicity depending on the height of mounting of the detector and the area of the room.

The reflectors may be constructed in any convenient manner. For example, the cone or half-cone may be machined from solid metal or pressed from metal sheet. Alternatively, the reflectors may be formed from a moulding of plastics covered with a surface layer of any suitable metal for reflecting the microwave beam.

The reflector may be of shapes other than conical to give different configurations for the beam coverage. Thus the reflector might be convex to increase the spread of the beam vertically as well as horizontally.

Claims (10)

1. A radar detector device comprising means for transmitting a microwave signal beam, means for receiving signals reflected back from objects in the path of the beam, means for sensing, from the reflected signals, the presence of an object which is changing its distance from the device, and reflecting means disposed in the path of the signal beam and shaped to deflect the signal beam in a manner to increase the angle of divergence thereof, thereby increasing the area of the field covered by the beam.

2. A radar detection device according to claim 1, wherein the reflecting means has a continuous curved surface the generators of which are inclined at an angle to the axis of propagation of the beam.

3. A radar detection device according to claim 2, wherein said curved surface comprises part of the surface of a cone.

4. A radar detection device according to claim 3, wherein the cone of which the curved surface forms part has an axis which extends in the same direction as the axis of propagation of the beam.

5. A radar detection device according to claim 3 or4, wherein the angular extent of said curved surface with respect to the axis of propagation of the cone is substantially 1800.

6. A radar detection device according to claim 3 or 4, wherein the angular extent of said curved surface with respect to the axis of propagation of the cone is substantially 360 .

7. A radar detection device according to claim 3, 4, 5 or 6, wherein the reflecting curved surface forming part of the surface of a cone extends up to the apex of the cone.

8. A radar detection device according to any one of claims 3 to 7, wherein the semi-vertical angle of the cone is about 45".

9. A radar detection device according to any one of claims 3 to 7, wherein the semi-vertical angle of the cone is about 60 .

10. A radar detection device substantially as hereinbefore described with reference to the accompanying drawings.